The present invention relates to apparatus for manufacturing of integrated circuits.
One of the basic problems in integrated circuit manufacturing is particulates. This problem is becoming more and more difficult, because of two trends in integrated circuit processing: First, as device dimensions become smaller and smaller, it is necessary to avoid the presence of smaller and smaller particles. This makes the job of making sure that a clean room is really clean increasingly difficult. For example, a clean room which is of class 1 (has one particle per cubic foot) for particles of one micron and larger may well be class 1000 or worse if particle sizes down to 100 angstroms are counted.
Second, there is increased desire to use large size integrated circuit patterns: for example, integrated circuit sizes larger than 50,000 square mils are much more commonly used now than they were five years ago.
Thus, particulates are not only an extremely important source of loss in integrated circuit manufacturing, but their importance will increase very rapidly in the coming years. Thus, it is an object of the present invention to provide methods for fabricating integrated circuits with reduced particulate contamination.
A common problem with prior art plasma etching machines has been particulate generation. Typically plasma etching stations are among the worst stations for depositions of particulates on the sensitive active area of the wafers. One particularly difficult area is the use of "shower head" gas distribution systems, where gas is distributed to the face of the wafer through multiple holes in the face of the active electrode. While this arrangement would seem to to be an attractive way to provide a highly uniform distribution of available process gasses over the face of the wafer, in practice, polymeric or other byproducts of the plasma phase reactions tend to deposit in the holes in the shower head, providing particulates which can be blown through and then immediately deposited on the wafer. Other arrangements for supplying process gasses have their own problems; problems of nonuniformity are common to many gas supply arrangements.
A novel teaching of the present invention is that the flow of gasses to the wafer face should be dominated by diffusion. That is, this aspect of the present invention provides a low-pressure plasma etching (or reactive ion etching) station wherein there is no bulk gas flow over the wafer face. That is, the feed gas species (which are needed for the plasma reactions which will produce the desired ions and free radicals to actually effect etching) are transported into the high-field region close to the face of the plasma (where they will be available for dissociation), not by bulk flow of a gas stream, nor (preferably) even by turbulent eddy currents in a region where there is no overall average bulk flow, but by diffusion. This means that particulate transport by gas-flow-assisted transport to the face of the wafer is greatly reduced.
In a further feature of this plasma reactor, the feed gas distributor is made of an insulating material. The feed gas distributor can thus be positioned reasonably close to the wafer face (i.e. less than one wafer diameter away) to help assure uniform availability of the reaction gasses, but there will not be a large potential drop across the dark zone in the plasma immediately adjacent to the feed gas distributor (as there would be if the distributor were made of a conductive material), and thus deposition of polymeric or other plasma reaction products on the feed gas distributor will be reduced. This means that transport of these reaction products to the wafer face as particulates will also be reduced.
In a further feature of the invention, wafers are etched in a face down position, and a gas distributor is provided which is below the wafer face and has ports blowing away from the wafer. This helps to assure that the bulk gas flow is downward and away from the wafer face, and thus reduces the likelihood of transport of particulates to the wafer face. Preferably the vacuum (exhaust) port is below the distributor.
A further innovative feature taught by the present application is a plasma (or RIE) reactor for face down etching of wafers, wherein essentially all of the grounded metal reactor chamber walls seen by the plasma move as a unit to open and close the reactor. That is, a bellows (preferably vacuum tight) is provided which supports the counter electrode opposite the wafer, and also supports the chamber sidewalls and (preferably) the process gas distribution distributor, so that all of these elements move as a unit. By reducing mechanical movements in proximity to the wafer, generation of particulates thereby is reduced.
The prior art of openable and closeable RIE reactors would typically use complex mechanical actuators, such as feedthroughs or cams, to clamp the wafers in place. However, in the present invention, the only moving mechanical elements are the wafer support pins, which are mounted on (and move only a short distance against the pressure of) an elastic support, so that the process chamber bottom portions can be closed against the powered electrode support when the plasma reactor is sealed to begin plasma etching.
Another innovative feature of this embodiment is that a quartz top layer is provided on the chamber housing, as the layer which will mate to the powered electrode support. This quartz top layer helps to preserve a high area ratio of powered electrode area to ground plane area in the chamber, which provides enhanced ion bombardment on the powered electrode, which is well known to those skilled in the art. This enhanced bombardment is desirable to assist in anisotropic wafer etching. The use of quartz here is further advantageous in that it is transparent to a wide variety of ultraviolet wavelengths, so that optical end point detection, and operator inspection of the plasma etching operation, are both facilitated.
The quartz top layer is also configured to provide excellent uniformity of ion bombardment. That is, the plasma is confined by quartz walls, for several centimeters away from the face of the wafer, to the shape of a cylinder having approximately the same width as the wafer. This collimation of the plasma provides improved uniformity of ion bombardment, which results in the advantage of more uniform etching. Walls made of quartz (or other high-temperature dielectric) are particularly well suited for this collimation, since they have less interaction with the plasma than metal walls would.
A further innovative feature is that a light bleed of helium is preferably provided to the back side of the wafer. This helium bleed assures good and uniform thermal contact between the wafer and the powered electrode under vacuum.
According to the present invention there is provided: An apparatus for plasma-assisted etching of integrated circuits, comprising: wafer supports for supporting a wafer containing partially fabricated integrated circuits face down, with the backside of said wafer adjacent to a powered electrode; a ground electrode positioned approximately in opposition to said powered electrode, said grounded electrode being supported by a base plate in fixed relation to reactor sidewalls; and a gas distributor which is fixed in relation to said grounded electrode; said grounded electrode and sidewalls being movable as a unit toward said powered electrode to effect a vacuum-tight seal between said powered electrode and sidewalls.
According to the present invention there is also provided: A plasma reactor comprising: supports shaped to hold a partially fabricated integrated circuit wafer face down; connections to communicate rf power to said wafer; and a gas distributor comprising orifices connected to exude gas in proximity to said water face, said gas distributor consisting essentially of a non-conductive material.
According to the present invention there is also provided: An apparatus for plasma-assisted etching of integrated circuits, comprising: wafer supports for supporting a wafer containing partially fabricated integrated circuits face down, with the backside of said wafer adjacent to a powered electrode; a ground electrode positioned approximately in opposition to said powered electrode, said grounded electrode being supported by a base plate in fixed relation to reactor sidewalls; and a gas distributor consisting essentially of a non-conductive material which is fixed in relation to said grounded electrode, said gas distributor comprising orifices connected to exude gas in proximity to said wafer face; said grounded electrode and sidewalls being movable as a unit toward said powered electrode to effect a vacuum-tight seal between said powered electrode and sidewalls.
According to the present invention there is also provided: A method for plasma-assisted etching of integrated circuit structures, comprising the steps of: supporting a wafer face down; supplying a flow of process gasses to a chamber contiguous with the underside of said supported wafer; and applying rf power to said chamber to generate a plasma in proximity to the underside of said wafer; wherein said process gasses are supplied using an orifice pattern and pressure such that transport of said process gasses (and of reaction products of said process gasses) to said face of said wafer is dominated by diffusion.